An image forming apparatus of the electrophotography type such as a laser printer forms images on recording mediums such as papers or OHP sheets by rotating a photosensitive drum to transfer toner adhering to a circumferential surface thereof to the recording mediums.
Accordingly, when transferring toner to a recording medium, in case the photosensitive drum does not rotate at a constant speed with respect to the recording medium, there is undesirably raised image defects such as periodical white stripes and dark stripes in a conveying direction of the recording medium.
In the following explanation, such record defect is referred to as banding, and a state in which the photosensitive drum does not rotate at a constant speed with respect to a recording medium is referred to as rotational irregularity of the photosensitive drum.
The rotational irregularity of the photosensitive drum is caused mainly by deteriorated precision of gears for rotating the photosensitive drum, such as single pitch error, pitch variation, normal pitch error, and tooth profile error of gears. The single pitch error, pitch variation, normal pitch error, and tooth profile error are prescribed by JIS (Japanese Industrial Standards) B 1702.
That is, since gears transmit rotational forces when the gears are rotated and teeth are engaged with teeth of another gear one after another, a load applied to each tooth fluctuates when the teeth are engaged one after another, and the load fluctuation causes the rotational irregularity.
Accordingly, when there are concurrently causes that distances between teeth are not constant and that tooth profiles are not precise, rotational motion cannot be transmitted accurately. Thus, the load fluctuation becomes significantly large when the teeth are engaged one after another. Therefore, instantaneous large rotational irregularity is generated. Hence, low precision of teeth causes banding having a cycle of the teeth of the gears.
When gears with high precision are used, the rotational irregularity can be reduced, and also the banding can be reduced. On the other hand, since gears transmit a rotational force when the teeth are engaged one after another, it is difficult to completely remove the rotational irregularity generated when the teeth are engaged one after another.
U.S. Pat. No. 6,142,690 (corresponding to Japanese Patent Application Publication No. HEI-10-312097) discloses increasing the number of teeth of gears configuring a gear mechanism to reduce intervals between bandings so that the banding cannot be easily recognized.
When the number of teeth of gears for rotating the photosensitive drum is increased, the number of teeth of gear which is rotated while a recording medium is conveyed by a unit length is also increased. Hence, teeth of a gear switch engagement with teeth of another gear in a shorter period (i.e., switching period becomes shorter and switching frequency becomes higher), making the banding less clearly recognized.